The invention proceeds from a process for producing a directionally solidified casting and from an apparatus for carrying out the process as is described, for example, in U.S. Pat. No. 3,532,155. The process described serves to produce the guide vanes and rotor blades of gas turbines and makes use of a furnace which can be evacuated. This furnace has two chambers which are separated from one another by a water-cooled wall and are arranged one above the other, the upper chamber of which is designed so that it can be heated and has a pivotable melting crucible for receiving material to be cast, for example a nickel base alloy. The lower chamber, which is connected to this heating chamber by an opening in the water-cooled wall, is designed so that it can be cooled and has walls through which water flows. A driving rod which passes through the bottom of this cooling chamber and through the opening in the water-cooled wall bears a cooling plate through which water flows and which forms the base of a casting mould located in the heating chamber.
When carrying out the process, first of all the alloy which has been liquefied in the melting crucible is poured into the casting mould located in the heating chamber. A narrow zone of directionally solidified alloy is thus formed above the cooling plate forming the base of the mould. As the casting mould is moved downwards into the cooling chamber, this mould is guided through the opening provided in the water-cooled wall. A solidification front which delimits the zone of directionally solidified alloy migrates from the bottom upwards through the entire casting mould, forming a directionally solidified casting.
A further process for producing a directionally solidified casting is disclosed in U.S. Pat. No. 3,763,926. In this process, a casting mould filled with a molten alloy is gradually and continuously immersed into a tin bath heated to approximately 260° C. This achieves a particularly rapid removal of heat from the casting mould. The directionally solidified casting formed by this process is distinguished by a microstructure which has a low level of inhomogeneities. When producing gas turbine blades of comparable design, it is possible using this process to achieve α values which are almost twice as high as when using the process according to U.S. Pat. No. 3,532,155. However, in order to avoid unwanted gas-forming reactions, which can damage the apparatus used in carrying out this process, this process requires a particularly accurate temperature control. In addition, the wall thickness of the casting mould has to be made larger than in the process according to U.S. Pat. No. 3,532,155.
U.S. Pat. No. 5,168,916 discloses a foundry installation designed for the fabrication of metal parts with an oriented structure, the installation being of a type comprising a casting chamber communicating with a lock for the introduction and extraction of a mould, via a first opening sealable by a first airtight gate apparatus for casting and for cooling the mould placed in the chamber. In accordance with the invention, the installation includes, in addition, a mould preheating and degassing chamber communicating with the lock via a second opening sealable by a second airtight gate.
U.S. Pat. No. 5,921,310 discloses a process which serves to produce a directionally solidified casting and uses an alloy located in a casting mould. The casting mould is guided from a heating chamber into a cooling chamber. The heating chamber is here at a temperature above the liquidus temperature of the alloy, and the cooling chamber is at a temperature below the solidus temperature of the alloy. The heating chamber and the cooling chamber are separated from one another by a baffle, aligned transversely to the guidance direction, having an opening for the casting mould. When carrying out the process, a solidification front is formed, beneath which the directionally solidified casting is formed. The part of the casting mould which is guided into the cooling chamber is cooled with a flow of inert gas. As a result, castings which are practically free of defects are achieved with relatively high throughput times. However, the quality of complex shaped castings such as turbine blades and vanes with protruding geometrical features, e.g. a shroud, platform or fin, will suffer from a heat flux which is not aligned to the vertical withdrawal direction, when the flow of inert gas impinges on such protruding features causing an excessive cooling due to the steep increase in outer surface area associated with a protruding feature. In directionally solidified polycrystals (DS) this causes undesired inclined DS grain boundaries, and for both, DS and single crystal (SX) articles the risk for undesired stray grains is increased. Furthermore, the vector component of the thermal gradient which is aligned to the vertical withdrawal direction is decreased, as a portion of the heat flux is not aligned with the vertical direction and therefore does not contribute to establish the vertical thermal gradient. Consequently the process does not achieve an optimum thermal gradient in vertical direction and therefore there is a risk for undesired freckles (chain of small stray grains, which may occur in particular in thick sections of a casting). Furthermore, the dendrite arm spacing is roughly inversely proportional to the square root of the thermal gradient, so the dendrite arm spacing is increased by decreasing the thermal gradient. This means that the distance from a dendrite stem to an adjacent interdendritic area is increased, which increases the amount of interdendritic segregation (e.g. diffusion has to overcome a larger distance). This may cause undesired incipient melting during a subsequent solutioning heat treatment, which is required for almost all of today's Nickel-base SX and DS superalloys. Additionally, an increased dendrite arm spacing increases the interdendritic spaces, where pores may form, and therefore causes an undesired increase in pore size.